Ablative nozzle



United States Patent O 3,309,874 ABLATIVE NOZZLE Bert B. Gould, 669 Vistamont, Berkeley, Calif. 94708 Filed Feb. 4, 1965, Ser. No. 431,781 Claims. (Cl. 60--253) This invention relates to a nozzle structure for a miniature rocket and more particularly relates to an ablative nozzle.

This Iapplication is a continuation-in-part of my copending application Ser. No. 352,998, filed Mar. 18, 1964, and entitled, Ablative Nozzle, which application is a continuation-in-part of my then copending application Ser. No. 93,210, liled Mar. 3, 1961, and now abandoned. This application is the same as the parent application Ser. No. 93,210 and may be considered to be the equivalent of a continuation thereof.

Miniature rockets have been described which are effective small arms Weapons. A typical miniature rocket is about one-tenth inch in diameter and about one and onehalf inches long.

In order to provide maximum efficiency, particularly with small rockets, it is important that the burning pressure be maintained reasonably uniform during the entire burning period. In large rockets designed to travel in the thin or absent air of the upper atmosphere, larger diameters and correspondingly higher drag ratios are of minor importance, and it is impractical to secure desired burning characteristics by various propellant perforations such as by forming the grain in the shape of a rod, rod and surrounding tube, star or the like so that it is possible to obtain constant or even regressive burning areas. However, both because of the requirement for minimum drag and therefore minimum cross-sectional area and because of the small size and mechanical manufacturing difficulties, these involved perforations become impractical with miniature rockets and the only practical grain configuration known at the present time is the use of a single cylindrical burning perforation. Such miniature rockets can be made by placing a mandrel in the center of the casing and pouring the liquefied propellant therein whereupon the mandrel can be withdrawn leaving the single grain with a round perforation in the center. Obviously, such a grain configuration will 'burn progressively and the burning surface will enlarge proportionally to the open diameter.

Enlargement of the area evolving gas increases the amount of gas that must exit through the nozzle throat, which resistance increases the pressure in the chamber. The rate at which a propellant burns, however, depends upon this chamber pressure, and may be approximated by Vieilles law r=aPn where r is the rate of burning, a is a constant, P is the chamber pressure, and n is a number characteristic of the particular propellant, for example 0.6 for certain doublebase propellants. In consequence, the chamber pressure varies with the area of burning propellant and the area of nozzle throat in accordance with the following relationship wherein Ab is the varea of burning propellant and A, is the area of the nozzle throat. Thus, if the area increases by a factor of 3 (which is typical in small rockets) while the throat area is maintained constant, the chamber pressure will be increased by a factor of 15.6. This means that a heavy case must be used since the case must contain the pressure at its highest point. Thus, the case must Patented Mar. 21, 1967 ice be much heavier than is necessary during the initial burning stages of the rocket reducing the efficiency of the rocket.

It has now been discovered that this disadvantage can be largely offset by the use of an ablative nozzle. As burning progresses, the nozzle is boiled or eroded away so that its throat area becomes larger. If during the burning the throat ablates and increases in area by a factor of only 2, the chamber pressure would increase by a factor of 2.8, rather than 15.6 in the absence of ablation, thus maintaining a more constant pressure within the rocket and permitting a lighter weight case for a given propellant cha-rge. In other words, during the initial burning of the rocket when the ra-te of gas evolution is relatively low, the nozzle is small to produce a high pressure within the chamber while as the rocket burns and the rate of gas evolution increases, the area of the throat also increases thus tending to maintain a constant chamber pressure.

Generally speaking, the objects of this invention are achieved by employing a nozzle of an ablative material such as a light metal or a plastic composition. Ablation of aluminum or magnesium nozzles may be controlled by adjusting the proportion of oxidizer in the propellant composition, while with plastic nozzlesvit is convenient to change the plastic composition. For example, the sequence: polyethylene, nylon,'Teflon, Bakelite is in order of increasing resistance to ablation, and the addition of a filler such as glass fiber or asbestos will substantially increase the resistance of each of these.

The shape of the nozzle also influences the rate of ablation; for example, it will be faster if the divergent section has the shape of a parabola rotated on its axis than the shape of a cone. Thus there is shown in FIGURE 7 a nozzle structure 23 wherein the divergent section is in the form of a parabola 25. The dot-dash line 27 represents the shape of the divergent section if it were in the form of a cone. The dotted line represents the parabola if it were continued. Even the shape of the nozzle may be controlled during ablation by laminating a less resistant plastic composition on the surface of one of greater resistance, so that at burn-out the nozzle will have acquired substantially the shape of the more resistant composition.

In the drawings forming part of this application:

FIGURE l is a sectional view of a rocket embodying the present invention before the rocket is tired;

FIGURE 2 is a section on the line 22 of FIGURE 1;

FIGURE 3 is a section on the line 3 3 of FIGURE l;

FIGURE 4 is a view of the rocket of FIGURE 1 showing the parts just before burn-out;

FIGURE 5 is a section on the line 5 5 of FIGURE 4;

FIGURE 6 is a section on the line 6-6 of FIGURE 4; and

FIGURE 7 is an enlarged diagram of a nozzle throat.

Referring now to FIGURES 1-6 by reference characters, there is shown a rocket having a casing 7 with a propellant grain 9 therein. The rocket is also provided with a nozzle 11 having at least a throat made of an ablative material. The grain 9 has a round central port 13. It will be seen from FIGURE 2 that the area 15 available for burning is relatively small while in FIGURE 5 the area 17 is considerably increased. The nozzle area 19 is relatively small before the rocket is ignited but increases to a relatively large size at at 21 as the rocket burns. Although the nozzle area increases in size, the increase is not as great as the increase in burning area as is shown.

I claim:

1. A miniature rocket having an elongate casing, a propellant grain lining the interior of the casing with a perforation extending lengthwise through the central portion of the grain, and a nozzle on the end of the casing in axial alignment with the perforation, said nozzle having a restricted throat portion which provides a restricted passage 3 in axial alignment with the perforation, said throat portion being formed of ablative material which in crosssection varies in composition from the inside out with the material making up the inne-riportion ablating more rapidly than the material making up the outer portion as the combustion gases pass through the nozzle whereby the rate of ablation decreases during burning of the grain.

2. A miniature rocket as claimed in claim 1 in which the ablative throat portion is formed of a plastic organic resinous material with the plastic material forming the inner portion being more susceptible to ablation by heat than the plastic material forming the outer portions of the throat.

3. A miniature rocket as claimed in claim 1 in which the throat portion is formed of metal selected from the group consisting of aluminum and magnesium, in which the metal contains an oxidizing agent present in higher concentrations 'at ,the inner portions of the metal throat than at the outer portions of the metal throat.

4. A miniature rocket having an elongate casing, a propellant grain lining the interior of the casing with a perforation extending lengthwise through the central portion of the grain, and a nozzle on the end of the casing -in axial alignment with the perforation, said nozzle having a restricted throat portion which provides a restricted passage in axial alignment with ,the perforation, said th-roat portion being formed of an ablative material which in cross-section from the inside out increases in resistance to ablation by the combustion gases passing through the nozzle from the burning grain whereby the rate of ablation decreases during burning of the grain and in which the throat portion is formed of a plastic organic resinous material lled with a relatively heat-insensitive inorganic filler material in which the amount of ller in the inner portion of the throat is less than the amount of ller in the outer portion.

v 5. A miniature rocket as claimed in claim 4 in which the filler in admixture with the plastic organic resinous material comprises glass libers.

References Cited by the Examiner UNITED STATES PATENTS 1,901,852 3/1933 Stolfa et al. 2,206,057 7/ 1940 Skinner. 2,952,972 9/1960 Kimmel et al 60-35.6 3,073,111 1/1963 Hasbrouck 60-35.6 v3,081,705 3/1963 Warnken 60-35.6 X

3,135,297 6/1964 Nordberg et al.

CARLTON R. CROYLE, Primary Examiner. 

1. A MINATURE ROCKET HAVING AN ELONGATE CASING, A PROPELLANT GRAIN LINING THE INTERIOR OF THE CASING WITH A PERFORATION EXTENDING LENGTHWISE THROUGH THE CENTRAL PORTION OF THE GRAIN, AND A NOZZLE ON THE END OF THE CASING IN AXIAL ALIGNMENT WITH THE PERFORATION, SAID NOZZLE HAVING A RESTRICTED THROAT PORTION WHICH PROVIDES A RESTRICTED PASSAGE IN AXIAL ALIGNMENT WITH THE PERFORATION, SAID THROAT PORTION BEING FORMED OF ABLATIVE MATERIAL WHICH IN CROSSSECTION VARIES IN COMPOSITION FROM THE INSIDE OUT WITH THE MATERIAL MAKING UP THE INNER PORTION ABLATING MORE RAPIDLY THAN THE MATERIAL MAKING UP THE OUTER PORTION AS THE COMBUSTION GASES PASS THROUGH THE NOZZLE WHEREBY THE RATE OF ABLATION DECREASES DURING BURNING OF THE GRAIN. 